The present invention relates to testing of storage batteries. More specifically, the invention relates to evaluating stored charge in an electrochemical cell or battery.
Stored charge is an important parameter in many applications of electrochemical cells and batteries. With traction batteries, stored charge represents an electric vehicle's fuel supply and thus determines how far the vehicle can travel before recharging. With stationary standby batteries, the level of stored charge determines how long a critical load can continue to function in the event of a power failure or disconnection from the ac mains. In automotive applications, stored charge determines the length of time that the lights and accessories can be operated when the engine is off, or when the charging system has malfunctioned.
With lead-acid batteries, relative stored charge, or state-of-charge (SOC), has been traditionally evaluated by observing either the battery's open-circuit voltage, or the specific gravity of the battery's electrolyte. However, neither of these measurements yields an absolute determination of the amount of stored charge. Furthermore, specific gravity measurements are messy and altogether impossible to perform on sealed lead-acid cells; and open-circuit voltage is difficult to determine under load conditions, and is imprecisely related to SOC since it is greatly affected by both "surface charge" and temperature.
Because of these problems, several techniques for correcting the voltage of lead-acid batteries to obtain SOC have been proposed. These include the techniques described by Christianson et al. in U.S. Pat. No. 3,946,299, by Reni et al. in U.S. Pat. No. 5,352,968, and by Hirzel in U.S. Pat. No. 5,381,096. However, such voltage correction methods are not very accurate. Furthermore, with electrochemical systems other than lead-acid, they may be of little help since battery voltage often bears very little relationship to stored charge.
Because of the problems with traditional methods for determining relative charge (SOC), many techniques based upon measuring ac impedance have been suggested. For example, U.S. Pat. No. 3,984,762 to Dowgiallo purports to determine SOC directly from the phase angle of the complex impedance at a single frequency. In U.S. Pat. No. 4,743,855, Randin et al. assert that SOC can be determined from the argument (i.e., phase angle) of the difference between complex impedances measured at two different frequencies. Bounaga, in U.S. Pat. No. 5,650,937, reportedly determines SOC from measurements of the imaginary part of the complex impedance at a single frequency. Finally, Basell et al., in U.S. Pat. No. 5,717,336 purport to determine SOC from the rate of change of impedance with frequency at low frequency. However, the fact that none of these ac impedance methods has gained wide acceptance suggests that they may not be altogether satisfactory methods for determining SOC.
The absolute stored charge or amp-hour capacity of batteries has been traditionally measured by timed-discharge tests. However, because of the expense and the time involved in performing such tests, ac techniques for determining amp-hour capacity have been proposed. Sharaf, in U. S. Pat. No. 3,808,522, teaches a method for determining the capacity of a lead-acid battery from measurements of its ac internal resistance. Yang, in U.S. Pat. No. 5,126,675, also uses measurements of internal resistance to determine battery capacity. Muramatsu reports, in U.S. Pat. No. 4,678,998, that he can determine both the remaining amp-hour capacity and the remaining service life of a battery from measurements of the ac impedance magnitude at two different frequencies. Fang, in U.S. Pat. No. 5,241,275, teaches a method for determining remaining capacity from complex impedance measured at two or three frequencies in the range from 0.001 to 1.0 Hz. Finally, Champlin, in U.S. Pat. No. 5,140,269, has shown that percent capacity can be determined from the measured dynamic conductance at a single frequency if the dynamic conductance of a reference, fully-charged, identically constructed, new battery is known. This method, although quite accurate, requires that aprioi data be available.